Modified rubber polymer latex

ABSTRACT

Process for forming a modified polymer having unsaturated carbon/carbon bonds including taking or making a colloidal dispersion of the polymer in an aqueous medium, swelling the polymer in the colloidal particles with at least one polymerizable monomer having a solubility in water of less than 10 −3  molar, and inducing a free-radical polymerization of the monomer(s) within the swollen colloidal particles of the polymer such that the monomer enters into a grafting reaction with the polymer. The process is particularly appropriate for preparation of modified natural rubber latex.

TECHNICAL FIELD

The present invention relates to modified polymers of the type includingcarbon/carbon double bonds. More particularly the invention relates tosuch polymers when modified by the polymerisation of at least onemonomer of low water solubility within a pre-existing polymer latex toyield materials containing interlinked polymers.

BACKGROUND ART

Modification of rubbery polymer colloids containing double bonds iscommon, including for example post-polymerisation processing ofstyrene-butadiene rubber and of neoprene. Another example comprisesmodification of natural rubber latex. One of the major components innatural rubber latex is cis-1,4polyisoprene, of which certain propertiesare unmatched by most synthetic rubbers. In addition, the unsaturatedbackbone in natural rubber allows various types of chemical modificationto be performed, to yield a wide range of rubber products found intoday's market. Prior chemical modifications of the isolated polymerinclude hydrogenation, epoxidation, chlorination, grafting, vulcanisingand other polymerisation processes. Modification of a rubber latex (ie.,without prior isolation of the polymer) is more challenging. Typical ofearlier studies are those carried out using natural rubber latex swollenwith methyl methacrylate (MMA) and then polymerised. The resultingpolymer consists in part of PMMA grafted onto natural rubber,commercially known as Hevea Plus, a product which has now beendiscontinued due to low levels of grafting and cracking on casting. Manyattempts have been made by other investigators to graft oil-solublevinyl monomers onto natural rubber latex using various types ofinitiators. However, the problems associated with their attempts rangefrom secondary particle formation, types of grafting (ie. comb orT-shape) to non-uniform modification of all rubber chains. All currentproducts are spatially non-uniform: eg., electron microscopy shows thatthe particles have spatially separate regions comprising pre-existingpolyisoprene and new polymer.

DISCLOSURE OF INVENTION

The aim of this work is to modify rubber latices including those ofnatural rubber to produce novel materials in order to overcome some ofthe problems stated above. Experiments based on a mechanisticunderstanding of emulsion polymerisation were designed, which resultedin the use of monomers of very low water solubility, such as vinylneo-decanoate. The inventors characterise the morphology and propertiesof the resulting materials as containing interlinked polymers. The term“interlinked” in this context is taken to mean physical and/or chemicallinking between polymers.

The present invention consists in a process for the formation of amodified polymer of the type having unsaturated carbon/carbon bonds, theprocess including the steps of:

a) taking or making a colloidal dispersion of the polymer in an aqueousmedium,

b) swelling the polymer in the colloidal particles with at least onepolymerisable monomer having a solubility in the aqueous medium of lessthan 10⁻³ molar, and

c) inducing a free radical polymerisation of the monomer(s) within theswollen colloidal particles of the polymer.

In another aspect the present invention consists of a material in theform of interlinked polymers produced by the method according to thepresent invention. In a still further aspect the present inventionconsists of a polymer of the type having unsaturated carbon/carbonbonds, wherein after polymerisinig an added monomer or monomers there isinterlinked with the original polymer a new polymer or polymers formedin-situ within the original polymer from the added monomers.

The present invention is particularly applicable to the preparation ofmodified natural rubber latex. In this embodiment of the invention thenatural rubber latex, which is already in the form of a colloidalsuspension of polymer particles in water, is swollen with the monomer(s)and then polymerised. The present invention may with proper selection ofreagents, however, be used to prepare modified polymers based on anypolymer including unsaturated carbon/carbon bonds which may be formedinto or exist as a colloidal suspension. Among the synthetic elastomersto which the present invention may be applied are isoprene rubbers,chloroprene rubbers including neoprene rubbers, polybutadiene rubbers,nitrile-butadiene rubbers, styrene-butadiene rubbers, polypentenamers,and ethylene-propylene-diene terpolymers. The unsaturated carbon/carbonbonds are usually double bonds in or pendant from an aliphatic chain butare not necessarily so.

The monomers for use in the present invention are characterised by theirlow solubility in aqueous media. They should have a solubility in waterof less than 10⁻³ M and preferably between 10⁻⁴ M and 10⁻⁹ M, morepreferably between 10⁻⁵ M and 10⁻⁷ M. To be useful in carrying out thepresent invention they must be capable of swelling the polymer and alsobe capable of free-radical initiated polymerisation. The most preferredmonomer for use in carrying out the present invention is vinylneo-decanoate. Other monomers that are preferred are straight chain orbranched chain alkyl or aryl vinyl esters having at least 6 carbonatoms, alkyl acrylates, alkyl methacrylates, vinyl acrylates and vinylmethacrylates. More preferred are vinyl 2-ethylhexanoate, vinyl 4-tertbutyl benzoate, n-dodecyl acrylate, n-dodecyl methacrylate, 2-ethylhexylacrylate, p-methyl styrene, vinyl toluene and 4-tert butyl styrene. Themonomers are preferably formed into an emulsion with a suitablesurfactant and added to the dispersion of the polymer or they are addedto the dispersion and agitated. It is normal to allow the monomer(s) tosit in contact with the polymer for a period of from minutes to somehours in order for the monomer(s) to penetrate the polymer and swell it.The monomer(s) should for preference not be added in a greater amountthan can be adsorbed by the polymer. Provided one or more of thelow-solubility monomers described above are present, other monomers ofany solubility may also be added to the reaction mixture and included inthe polymers according to the present invention.

The term swelling in this context is taken to mean the entry ofmonomer(s) into the polymer particle and diffusion through the particle.There will usually be a concomitant physical increase in the size of thepolymer particles.

Without limitation to the scope of the present invention the followingexplanation of the mechanism of the present invention is proposed. Theprocess seeks to avoid the formation of secondary particles during thepolymerisation process. It is thought that secondary nucleation resultsin the formation of new particles outside the polymer particles, and thesubsequent imbibing of these new particles into pre-existing polymerparticles is responsible for some or all of the spatial homogeneitiesobserved in current products. The avoidance of secondary particleformation, including particles which may be later imbibed intopre-existing polymer particles, was carried out by choosing monomers ofvery low water solubility. This is because radicals formed frominitiator are most likely to react with an individual monomer molecule,perhaps in the water phase to form a species that is subsequently freeto polymerise anywhere in the particle. It is believed that if thepolymer were present in the aqueous phase in a greater concentrationthere would be a reaction between the activated monomer molecule and oneor more other monomer molecules resulting in secondary particleformation. The products of the present invention show a number ofdistinct glass transition temperatures. These temperatures often arevery close to but not identical to the values that would be expectedfrom the pure polymers. The fact that the glass transition temperatureshave moved slightly suggests that in addition to the polymerisationreaction the activated monomer species may also enter into some graftingreactions with the polymer. During polymerisation some grafting to thecarbon-carbon double bonds and/or transfer reactions in the pre-existingpolymers may occur, which because it will commence early in thepolymerisation will interlink newly formed polymer into the pre-existingpolymer.

MODES FOR CARRYING OUT THE INVENTION

Natural rubber latex was diluted with water and surfactant, such asAerosol MA 80 or potassium oleate, to give the required solids content.Monomer in a ratio of 1:2 by mass of polymer to monomer (unlessotherwise stated) was then added and allowed to swell into the naturalrubber particle before commencement of polymerisation with awater-soluble initiator.

Analysis of the resulting materials to determine glass transitiontemperatures was carried out on a Modulated Differential ScanningCalorimetry (MDSC), using sample sizes of approximately 15 mg with aramp rate of 3° C./min from −120° C. to 120° C., or by dynamicmechanical thermal analysis (DMTA), using samples made from a film, forexample by casting a film on a glass plate, scanning for example from−90° C. to room temperature with an applied frequency of 1 Hz. Particlemorphology was characterised by Transmission Electron Microscopy (TEM)using two techniques: cryo-sectioning and also microtominig the sample(set in a resin) with OsO₄ staining.

Results and Discussion

EXAMPLE 1

(Vinyl Neo-decanoate+NRL)

The polymerisation of vinyl neo-decanoate into the natural rubber latexwas performed at 50° C. It was found that the polymerisation did notexceed above 30% conversion after a day. Kinetic analysis confirmed thatthe mechanism for this retardation was due to vinyl neo-decanoate chainend radicals preferentially abstracting hydrogens from the backbone ofpolyisoprene to form stable allylic radicals. These incipient radicalsare slow to react with vinyl neo-decanoate monomer, which slows the rateof polymerisation drastically, and therefore act as radical terminatorsfor polymeric radicals. This results in an increase in chain branchingto the polyisoprene backbone.

Analysis of the resulting material by MDSC gave two distinct glasstransition temperatures (Tg's): the first at −61° C. and the second at−7° C. The first Tg of −62° C. corresponds closely to that of purenatural rubber (Tg=−67° C.), whereas the second corresponds to that ofpoly vinyl neo-decanoate (Tg=−7° C.). Particle morphology appears to berelatively homogeneous as observed from transmission electron microscope(TEM) on the cryo-sectioned sample, and in addition showed no secondaryparticle formation. This suggests that the material contains interlinkedpolymers. This is in contrast to products formed by conventionalsecond-stage polymerisation of natural rubber latex where thesecond-stage polymer is seen to be not uniformly distributed or blendedthroughout the pre-existing particle.

EXAMPLE 2

(Vinyl Toluene+NRL)

The polymerisation of vinyl toluene into the natural rubber latex wasperformed at 50° C. and reached high conversions. MDSC analysis gave twodistinct glass transition temperatures, the first at −63° C. and thesecond at 91° C. The first transition corresponds closely to that ofpure natural rubber, whereas the second corresponds to that of polyvinyl toluene (Tg=93° C.). Once again, there was no secondary particleformation. as observed from TEM.

EXAMPLE 3

(n-Dodecyl Acrylate+NRL)

The polymerisation of n-dodecyl acrylate into natural rubber latex wasperformed at 50° C. and reached very high conversions. MDSC analysis onone sample so prepared gave two distinct glass transition temperatures,the first at −66° C. and the second at −4° C. The first transitioncorresponds closely to that of pure natural rubber, whereas the secondcorresponds to that of poly n-dodecyl acrylate (Tg=−4° C.). DMTAanalysis on the unpolymerised NRL shows a peak at −50° C.; a samplepolymerised as above, when scanned by DMTA between −90 and 10° C., showsa peak at −43° C., i.e., the rubber peak is shifted which indicatesgrafting and/or other form of interlinking. Once again, particlemorphology appears to be relatively homogeneous, with no secondaryparticle formation, as observed from TEM on the cryo-sectioned andmicrotomed sample.

EXAMPLE 4

(n-Dodecyl Acrylate+Vinyl Neo-decanoate+NRL)

The polymerisation of n-dodecyl acrylate and vinyl neo-decanoate intonatural rubber latex was performed at 30° C., and reached very highconversions. At a mass ratio of 1:0.99:0.052 for NRL: n-dodecylacrylate:vinlyl neo-decanoate, MDSC analysis gave several glasstransition temperatures, the first a clear peak at −66° C., the second aweak peak at −14.4° C., and the third a weak peak at −4° C. The firsttransition corresponds closely to that of pure natural rubber, whereasthe third corresponds to that of poly n-dodecyl acrylate (Tg=−4° C.).The second glass transition probably indicates grafting of polyn-dodecyl acrylate onto natural rubber, perhaps (but not necessarily)where the allylic radicals (see Example 1) can either add to n-dodecylacrylate or terminate polymeric radicals. When the amount of vinylneo-decanoate was increased to a mass ratio of 1:0.45:0.1 forNRL:n-dodecyl acrylate:vinyl neo-decanoate, MDSC analysis gave thefollowing glass transition temperatures below −10° C.: the first at −66°C. corresponding to natural rubber, the second at −26.4° C., the thirdat −19.29. The second and third glass transitions indicate grafting ofpoly n-dodecyl acrylate onto natural rubber via the vinyl neo-decanoategrafting agent. Particle morphology appears to be relatively homogeneousas observed from TEM on the cryo-sectioned and microtomed samples, withno secondary particle formation, as observed by TEM.

EXAMPLE 5

(n-Dodecyl Methacrylate+NRL)

The polymerisation of n-dodecyl methacrylate into natural rubber latexwas performed at 50° C., and reached high conversions. MDSC analysisgave two distinct glass transition temperatures, the first at −66° C.and the second a broad one with a maximum at −53° C. The firsttransition corresponds closely to that of pure natural rubber, whereasthe second corresponds to that of poly n-dodecyl methacrylate (Tg=−55°C.). Particle morphology appears to have subinclusiolns concentrated atthe edge of the natural rubber particle, as observed from TEM on thecryo-sectioned and microtomed samples, with no secondary particleformation, as observed by TEM.

EXAMPLE 6

(n-Dodecyl methacrylate+Vinyl neo-decanoate+NRL)

The polymerisation of n-dodecyl methacrylate and vinyl neo-decanoateinto natural rubber latex was performed at 50° C., and reached highconversions. TEM analysis showed that the amount of subinclusions in theparticle (as in Case 5) decreased substantially, becoming morehomogeneous in nature. This result suggests that vinyl neo-decanoate notonly acts as a grafting agent to the polyisoprene backbone in naturalrubber but produces grafting sites spread homogeneously throughout thenatural rubber particle, which in turn increases the homogeneous natureof n-dodecyl methacrylate in natural rubber.

SUMMARY

When vinyl neo-decanoate, n-dodecyl acrylate and n-dodecyl methacrylate(which each has a solubility in water of less than 0⁻⁴ molar) are usedin various combinations, spatially homogenous particles are observedfrom TEM micrographs. The glass transition temperatures correspondclosely to those of the homopolymers of the monomer and to naturalrubber; some shifts however may be observed. Additional Tg's indicatesome grafting. These data suggest that some interlinkinig has occurred,quite different from the very inhomogeneous and non-uniform productformed by conventional second-stage polymerisation of natural rubberlatex.

INDUSTRIAL APPLICATION

The novel homogeneously-modified rubber latex could be used inapplications such as pressure sensitive adhesives, gloves, aircrafttires and other rubber products. The invention is a unique method ofproducing a partially or fully homogeneous modified rubber latex withoutthe disadvantages of the second-stage polymer being spatiallyinhomogeneous, which may otherwise have adverse properties in many ofthe above uses.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. A process for the formation of a modified polymer of the type having unsaturated carbon/carbon bonds, the process comprising the steps of: (a) taking or making a colloidal dispersion of the polymer in an aqueous medium; (b) swelling the polymer in the colloidal particles with vinyl neo-decanoate; and (c) inducing a free radical polymerization of said vinyl neo-decanoate within the swollen colloidal particles of the polymer such that said vinyl neo-decanoate enters into a grafting reaction with the polymer.
 2. The process as claimed in claim 1 in which the polymer is selected from the group consisting of natural rubber latex, isoprene rubbers, chloroprene rubbers, polybutadiene rubbers, nitrile butadiene rubbers, styrene-butadiene rubbers, polypentenamers, and ethylene-propylene-diene terpolymers.
 3. The process according to claim 2 wherein said polymer is a neoprene rubber.
 4. A process according to claim 2 in which the polymer is natural rubber latex.
 5. A process according to claim 1 in which said monomer has a solubility in water of less than 10⁻³ M.
 6. A process according to claim 5 wherein the solubility in water of said monomer is between 10⁻⁴ M and 10⁻³ M.
 7. A process according to claim 6 wherein the solubility of said monomer in water is between 10⁻⁴ M and 10⁻⁷ M.
 8. The process according to claim 1 in which other monomers of any solubility are also employed to swell the polymer.
 9. A process as claimed in claim 8 in which the at least one monomer is selected from the group consisting of n-dodecylacrylate and n-dodecylmethacrylate.
 10. A modified polymer produced by the process of claim
 1. 11. A material in the form of interlinked polymers produced by the process as claimed in claim
 1. 